Electrically conductive films and method for producing same



Aug. 4, 1959 CLARK, SR

ELECTRICALLY CONDUCTIVE FILMS AND METHOD FOR PRODUCING SAME Filed Nov.20, 1953 INVENTOR LELAND B. CLARK SR.

ATTORNEYJ ELECTRICALLY CONDUCTIVE FILMS AND A METHOD FOR PRODUCING SAMEThe present invention relates to a method for producing electricallyconductive films on impervious surfaces of solids, to the coatedsurfaces and to articles employing the same. A particular application ofthe method of the invention is the production of cathode-envelopes forionization detection devices which are to contain a halogen in theionizablegas filling.

Ionization detection devices to which the present invention relates haveas their prototype the well-known Geiger-Mueller counter or tube. Thecounter comprises an envelope in which is sealed the anode and theionizable gas filling which generally is provided with an admixture of asmall proportion of a gaseous or vaporous quenching agent to render thecounter self-quenching. The envelope is usually made of metal and thusalso serves as the cathode for the counter. Polyatornic organiccompounds, such as alcohol, and the halogens, chlorine and bromine,

have been used as quenching agents. The halogens havev the advantagethat, unlike polyatomic organic com-- pounds, they will not permanentlybreak down into smaller molecules of no quenching activity under theaction of overvoltages in the counter. The halogens have thedisadvantage, however, of being reactive toward the metals out of whichthe usual cathodes are made and will combine therewith to reduce thecontent of free halogen in the counter. This reactivity offthehalogensmilitates against their use as quenching agents since the life of acounter depends upon the maintenance of an adequate level ofconcentration of the quenching agent therein.

It has been heretofore proposed to use vitreous envelopes forGeiger-Mueller counters in which the cathode is a halogen-resistant,non-metallic conductive film on the inner surface of the envelope. Theconductive film is obtained by spraying a solution of a metal halide inalcohol on the inner surface of the vitreous tube out of which theenvelope is made while the tube is maintained at a temperature of fromabout 400 to 700 C. Counter States, Patent a thickness variation ofabout 20 to 30% for the indi vidual film; For best'results, the cathodefilm should have a nniform resistance per unit length which would beprovided by a level thickness of the film. Spray application of thefilm-forming solution to the inner surface of a tube, the envelope ofGeiger-Muellercounter is tubes made up with a glass envelope on whichthe cathode film has been provided by the above spray method using tintetrachloride in alcohol as'the film-forming solution have givenunsatisfactory performance. The films possess an optical haze and havebeen found to contain a considerable amount of moisture as is evidencedby a milky,

bined with the particles of the film or adsorbed thereon. Released intothe ionizable gas filling as water vapor during operation of thecounter, it causes the slope of the plateau of the counter to beincreased to such an extent as to render the tube useless for accuratecounting puropalescent appearance of the film. This mois-, ture ispresent in bound form, either chemically comusually tubular in geometry,is not suited to producing a film of level thickness.

It is an object of the present invention to provide a method forproducing clear, moisture-free, electrically conductive films onimpervious surfaces of solids. It is a further object to provide amethod for producing a clear, moisture-free, halogen-resistant,electrically conductive fihn on the inner impervious surface of tubessuitable for forming envelopes for ionization conduction devices. It isa still further object to provide a method of this kind which isproductive of a level thickness in the film. It is another object toprovide solid articles having an impervious surface coated with a clear,moisture-free, halogen-resistant, electrically conductive film of levelthickness, especially tubes having an inner impervious surface coatedwith such a conductive film. It is also an object to provide ionizationdetection devices in which the envelope is coated on its inner surfacewith a clear, moisture-free, halogen-resistant cathode film.

become apparent hereinafter are accomplished in accordance with myinvention by a method which is relatively simple in its application tothe production on impervious surfaces of solids of a clear,moisture-free, halogenresistant, electrically conductive film of auniform thickness.

so, for example, vitreous, solids, such as glass and fused quartz,vitreous-surfaced solids, such 'as glazed ceramics and enamelledmetals,natural materials of little .or no porosity, such as mica and quartz,metals polished to a level or continuous surface, such as highlypolished steel, or which have been cast in smooth-faced molds, and metals of fine grain which as such or by virtue of having been worked havea level or continuous surface. a

In the practice of the method ,of my invention a dry aerosol comprisingthe product of mixing dry air in calculated proportions with the vaporsfrom the addition of a liquid metal polyhalide to dry methanol is passedover the impervious surface to be coated while the latter is tures whichwould cause sagging or other deformation of the same. The films whichcan be produced by the method of the invention are transparent, may beof various colors, and are formed of metal in a non-metallic state,m'ost'probably in the form of the finely crystalline higher oxide of-themetal which in the crystal lattice contains an excess form, mostprobably the metal. I

V The aerosol is formed in a dry atmosphere and the addition of-theliquid metal carried out in discrete fashion, suitablydropwise, where?by to control the rate of production of the vapors. Liquid Q metalpolyhalides' which maybe employed in the method are "antimonypentachlon'de, antimony pentafluoride;

polyhalide to the methanol is Patented Aug. 4, 1959 These and otherobjects of the invention which will By impervious surfaces as usedherein and in the; claims is meant one which is nonporous orsubstantially of the metalor oxygen in elemental germaniumtetrachloride, tin tetrachloride, titanium tetrachloride and vanadiumtetrachloride, of which tin tetrachloride constitutes a preferredspecies. The air employed in forming the aerosol may be dried in anysuitable fashion, for example, by passing it through a drying towerhaving a packing of porous ceramic tile covered with a body ofconcentrated sulfuric acid as the desiccant.

In the accompanying drawing forming part ofthe description of thepresent invention:

Figure l is a schematic showing of an apparatus suitable for carryingout the method of the invention, and

Figure 2 is a showing of a typical ionization detection device providedwith an envelope and cathode of the invention.

The apparatus shown in Figure l is designed as a closed system with theexception of an inlet for supply of the air to be dried and used in themethod and an outlet for discharge of aerosol exhaust from the system. Atube 1 serves as the air intake into the system and is connected forflow of the air into and through the drier 2 containing a packing ofporous ceramic tiles covered with a body of concentrated sulfuric acid,for example, 90% H 80 The drier may be of conventional design andprovided with feed and take off outlets (not shown) for replenishing thesupply of acid of drying strength. A tube 3 leads from the drier into amixing Zone 4 at the top of a glass vessel 5 which is designed to hold asupply of dry methanol. The vessel 5 is formed with a neck 6 at itsupper end and, as shown, has the configuration of the well-knownErlenmeyer flask. Arranged above the vessel 5 is a glass pipette 7 ofconventional construction for containing a supply of the liquid metalpolyhalide. The lower tube 8 of the pipette extends down through agasket 9 of rubber or of Tigon (an expansible plasic) into the neck 6 ofthe vessel 5 and terminates at a point which is below the juncture ofthe tube 3 with the neck 6 but is substantially above the bottom of thevessel 5. The tube 8 is tapered at its lower end from which the liquidmetal polyhalide is allowed to issue in the form of drops. A length ofrubber or expansible plastic tubing 10 is attached to the upper tube 11of the pipette 7 and is provided with a pinchcock 12thereon forcontrolling the rate of flow of the liquid from the tube 8. Extendingfrom the mixing zone 4 of the vessel 5 is a tube 13 which is coupled toa tubular mutfle 14 of an electrically heated muffle furnace 15 which iscapable of operating to heat the mufiie to operating temperatures forthe method. The size of the tubular 'mufiie 14 is so chosen that thediameter thereof is only a shade larger than that of the tube which isto be coated on its inner surface by treatment with the dry aerosol,whereby the latter can be slid into position in the tubular muffle andheld there by a tight fit. The muffle 14 is coupled with a tube 16 whichleads to a variable speed exhaust pump 17 which is provided with anexhaust discharge tube 18. The coupling between the tubular muffle 14and the adjacent tubes 13 and 16 is made outside of the furnace by meansof a gasket 19 of an expansible plastic such as Tigon. The tubesemployed in the apparatus should be of the same inner diameter in orderto promote easy flow of the air and aerosol in the system and may bemade of metal or glass.

For a more complete'undcrstanding of the invention, the method will bedescribedas it is carried out in an apparatus as shown in Figure 1. Atube 20 which is to be coated on its inner surface with the conductivefilm V is slid into position in the tubular muffle 14. This arrangementof the tube and muffie is illustrated in the drawing wherein the muflieis shown as being partly broken away in the area of the tube. The tube20 having been arranged in the muffle, the filrnace is set to heat toand maintain the temperature to be used during the operation of themethod. The tube and muffle having been brought to temperature, air isdrawn into the systemat 1 by operation of the variable speed exhaust 4pump 17. The exhaust pump is operated at low delivery rates, sufiicientto cause the air entering at the inlet 1 to move in gentle flow throughthe system. This gentle flow of air may be movement of the air at a ratejust suficient to cause it to progress through the system or one atrates which are moderately higher. Variation in the rate of flow of theair will depend upon the factors of travel time required for the air topass through the desiccant and come out in dry condition, theconcentration of the dispersed phase desired in the aerosol, and therate at which it is desired to deposit the film on the impervioussurface to be coated. The concentration of the film-forming or dispersedphase in the aerosol is not critical, the more dilute dispersions givinga slower rate of build-up of the film. The liquid metal polyhalide isfed from the pipette 7 through the tube 8 and enters in the form ofdrops into the body of dry methanol in vessel 5. The methanol is warmedby any suitable heating means (not shown) to promote reaction between it.and the entering liquid metal polyhalide, for example, to avtemperature in the region of 25 to 30 C. The rate of addition of thelatter is controlled by means of the pinchcock 12' on the expansibletubing 10 attached to the tube 11 of the pipette. By adjusting the rateof addition of the liquid metal polyhalide to the methanol, the volumeof the vapors generated can be controlled to vary the concentration ofthe film-forming or dispersed phase inithe aerosol for a set rate of airintake. The vapors from the addition of the liquid metal polyhalide tothe methanol rise into thezone 4 at the top of the vessel 5 and fromthere are swept out into the tube 13 by the stream of dry air enteringfrom the tube 3. Mixing of the dry air and vapors with production of thedry aerosol takes placein the tube 13. As a section of a continuouslyflowing fluid mass, the dry aerosol is moved into the hot mufiie andthrough the heated tube 20. In flowing over the inner surface of thetube 20, the areosol is caused to deposit a film of the conductivematerial thereon by the action of the applied heat. Flow of the aerosolover the inner surface of the tube is continued until the desiredthickness of film has been attained. Discharge of the spent or partiallyspent aerosol from the system is made through the exhaust pump 17 andexit tube 18; The snug fit' between the muffle 14 and the tube 20precludes deposition of a film on the outside surface of the tube.

The method of the present invention which operates from the gas phase todeposit the film on the impervious surface of the solid makes possiblethe production of conductive films which are of a uniform thickness inthe individual layer as shown by the reflection interference color ofthe film in white light. The color of the film is indicative of itsthickness and a single color therein of the uniformity in thickness. Theelectrical resistance of the film can be related to the color thereof.Thecolor of the film and, accordingly, its thickness will depend uponthe concentration of the dispersed phase in the aerosol, the rate offlow of the aerosol over the surface to be coated and the duration ofthe exposure of the surface to the aerosol. The optimum conditions forproducing any particular film color can be found by trial and error andonce determined can be readily duplicated.

Following the method and employing apparatus as described above, anelectrically conductive film was deposited 'on the inner surface of aglass tube using tin tetrachloride as the liquid metal polyhalide forthe production -of the aerosol. The tube was made of Pyrex glass No.7740 and measured 25 mm. in diameter and 16.25 cm. in length. Depositionof the film was made at 'a mufile temperature of 500 C. The pump wasoperated to induce a gentle flow of air through the system. The tintetrachloride was fed dropwise to 10 ml. of dry methanol warmed toapproximately 25 C., and at a rate such that approximately 2 ml. thereofwas added to the methanol over a period of 25 to 30 seconds. The coatingoperation having been completed within this time period (the operationis quite short), the pump was stopped and the mufiie and tube removedfrom the electric furnace, After having cooled down to room temperature,the treated glass tube was removed from the muffle. The inner surface ofthe tube was coated with a film for a length of 15 cm. The film wasclear, free from bound moisture, of a single color, indicating a uniformthickness, and non-metallic in appearance. Although the exact chemicalidentity of the film is not known, an analysis of such films hasdisclosed them to be formed of finely crystalline tin dioxide, of thenature of cassiterite, in which the crystallattice contains excess tinor oxygen in the elemental or uncombined form. The film is notchemically combined with the glass but is held thereon by some physicalforce or forces. The electrical resistance of the film, end to end ofthe tube, was 250 to 600 ohms. In this same way, films of the colorseries, gray, brown, lavender, blue and green which in the order namedhave a decreasing electrical resistance are readily produced in areas upto 18 square inches on the inner surface of the glass tube by modifyingthe rate of flow of the aerosol through the tube or the duration of theexposure of the tube to the flowing aerosol. Color is given herein asreflection interference color in white light. If the films have beenallowed to stand in the air whereby moisture has collected on thesurface of the films, they can be effectively dried to give satisfactoryperformance in the counter tube by wiping the surface with a softabsorbent material.

Films prepared as in the above example were evaluated for behavior asthe cathode in ahalogen-quenched Geiger-Mueller counter of the typeshown inFigureZ which is hereinafter described. A one mil wire ofplatinum-% iridium alloy was used as'the anode. The glass envelopescarrying the conductive film on the interior thereof were evacuated,subjected to positive ion bombardment, heated to 310 C. for to minutes,allowed to cool to approximately room temperature and then filled withneon at a' pressure of 5 50 (mercury) and chlorine at a pressure'of' 6mm. (mercury). A characteristic was run which showed that the tubes(envelope and film) started counting at 920 volts and had a good plateaufor 360 volts. The tubes can be filled with neon up to pressures of 700mm. (mercury) without showing a very large increase in starting voltage.In spite of the transparency of the envelope, the tubes showed noincrease in the counting rate on exposure to sunlight or to lightedmatches. This alone is a feature not shared by the conventional metalcounting tubes. The responseof the tubes along their axis showed uniformsensitivity. Over 9 cm. of the total active length of the tubes lies onthe flat portion of the sensitivity curve. These tubes are capable ofoperating at temperatures as high as 200 C. in contrast to theconventional counting tube which fails at 100 C. 'by the loss of theplateau. The dead time-recovery time envelope for a particular tubedesign can be readily shortened or lengthened by varying the magnitudeof the overvoltage applied to the tube.

In the Geiger-Mueller counter shown in Figure 2, the envelope 101 may bemade of glass or of other vitreous material or of a vitreous-surfacedceramic, such as porcelain. The tube from which the envelope is formedis sealed at the one end and provided with a pair of exterior supportinglugs through which the cathode and anode wires extend in gas-tightrelationship for connection with the counting circuit. The wire anode102 is made of halogen-resistant metal, suitably platinum10% in'diumalloy, and extends along the major or longitudinal axis of the envelope.The diameter of the wire anode may vary but should be substantially lessthan 50 mils and preferably is from about 1 to 10 mils. rAn anodediameter of 50 mils has failed to function properly in the tubes. Theanode wire is supported at its opposite end in known way by means of thehelical spring 103 to which it is spot-welded. The spring 103 istensioned against the opposed inturned, spaced ends 104 of the'env'elope and housed in the cap 105 which seals this end of theenvelope. The inner surface of the envelope carries a moisture free,halogen-resistant, electrically conductive film 106 produced thereon bymeans of the method of the invention while the envelope is in the formof a glass tube (open-ended envelope). This conductive film serves asthe cathode for the counter. A band 107 of colloidal platinum isprovided on the inner surface of the envelope 101 and by contiguitymakes electrical contact with the cathode 106, The band 107 is paintedon the inner surface of the tube in known way and prior to the coatingof the tube with the cathode film. A tungsten Wire 108 is electricallyconnected to the band 107 in known way, by spot welding to a platinumspring contact plate which presses onto the surface of the band, andpasses through the adjacent end of the envelope to serve as the cathodewire or lead. The atmosphere of the envelope 101 may be maintained atthe usual reduced pressures of from about 50m 60 millimeters (mercury)and is made up of an ionizable gas and a small proportion of a halogen.The ionizable gas is suitably neon or argon or mixtures of these rare ornoble gases. A typical gas filling for the envelope is an admixture ofneon 500 600 mm. (mercury) with chlorine 5-6 mm. (mercury).

The electrical circuit schematically shown in Figure 2 includes inaddition to the anode 102 and cathode 106, a recorder, such as anelectrometer, oscillograph, etc., for registering as a signal :theimpulse from the ionization produced in the gas-filled envelope 101 bythe received radiation. A'source of potential E is supplied in serieswith the gas-filled envelope to maintain the atmosphere therein at apotential gradient which will cause rapid production of ion pairs uponthe reception of the ioniz ing radiation. Resistance R and capacitance Cin series completes the circuit from the gas-filled envelope totherecorder.

The application of the teachings of the present invention is productiveof counter tubes (envelope and cathode) which have an extended operatinglife using halogen quenching agents and which may be repeatedly throwninto violent discharge without affecting the normal tube operation withits even pulse heights. An interesting observation in the case of thetubes which have a clear glass envelope, is to view the anode wire inthe tube in approximately darkened surroundings while the tube isoperating normally. Due to the transparency of the cathode and envelope,one will observe that'the ion sheath (Townsend Avalanche) about theanode wire is glowing and that its apparent glow intensity is quitesensitive to change in the amount of radiation falling upon the tube.This requires only a very simple circuit, i.e., the counter tube, aseries resistance and a high voltage source.

Various changes and modifications may be made in the invention hereindescribed without departing from the spirit or scope thereof. While Ihave described my invention with reference to the production ofmoisturefree, non-metallic cathode films in ionization tubes, it will beobvious that the gas phase deposition method of the invention is ofgeneral application for the production of moisture-free, non-metallic,electrically conductive films of level thickness on impervious surfacesof various heatresistant solids.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What I claim is:

1. A method of coating an impervious surface of a heat-resistant solidwith a clear, moisture-free, electrically conductive film whichcomprises, forming a dry aerosol comprising the product of mixing dryair with the vapors from the addition of a liquid metal polyhalide todry methanol and flowing the dry aerosol over the impervious surfacewhile it is maintained under dry conditions and is at a temperaturesufficient to cause deposition from the aerosol and formation of thefilm thereon.

2. The method as defined in claim 1, wherein the liquid metal polyhalideis tin tetrachloride.

3. A method of coating the inner impervious surface of a tube ofheat-resistant material with a clear, moisturefree, electricallyconductive film which comprises, form ing a dry aerosol comprising theproduct of mixing dry air with the vapors from the addition of a liquidmetal polyhalide to dry methanol and flowing the dry aerosol through thetube while the latter is maintained under dry conditions and is at atemperature suflicient to cause deposition from the aerosol andformation of the film thereon.

4. The method as defined in claim 3, wherein the tube is made ofvitreous material.

5. A method of coating the inner surface of a glass tube with a clear,transparent, moisture-free, electrically conductive film whichcomprises, forming a dry aerosol comprising the product of mixing dryair with the vapors from the addition of tin tetrachloride to drymethanol and flowing the dry aerosol through the tube while the latteris maintained under dry conditions and is at a temperature of from about300 to 700 C.

6. A heat-resistant solid having a vitreous surface coated with a clear,transparent, moisture-free, halogenresistant, electrically conductivefilm of a finely crystalline metal oxide, said film being of uniformthickness as shown by its single reflectance interference color in whitelight.

7. A ceramic tube having a vitreous inner surface coated with a clear,transparent, moisture-free, halogenresistant, electrically conductivefilm of a finely crystalline tin oxide, said film being of uniformthickness as shown by its single reflectance interference color in whitelight.

8. A vitreous tube having the inner surface thereof coated with a clear,transparent, moisture-free, halogenresistant, electrically conductivefilm of a finely crystalline tin oxide, said film being of uniformthickness as shown by its singlereflectance interference color in whitelight. v

9. A vitreous tube as defined in claim 8, wherein the tube is made ofglass.

10. An. ionization device comprising a rigid tubular envelope having animpervious inner surface coated with a clear, transparent,moisture-free, halogen-resistant, electrically conductive film of afinely crystalline metal oxide, said film being a cathode and of uniformthickness as shown by its single reflectance interference color in whitelight, an anode ofsmall diameter and an ionizable, halogen-containinggas filling in said envelope.

11. An ionization device comprising a vitreous tubular envelope coatedon the inner surface with a clear, transparent, moisture-free,halogen-resistant, electrically conductive film of a finely crystallinetin oxide, said film being a cathode and of'uniform thickness as shownby its single reflectance interference color in white light, an anode ofsmall diameter and an ionizable, halogen-containing gas filling in saidenvelope.

12. An ionization device as defined in claim 11, wherein the vitreoustubular envelope is made of glass.

13. An ionization device as defined in claim 12, wherein the halogen ischlorine.

14. A method of coating the inner surface of a glass tube with a clear,transparent, moisture-free, electrically conductive film whichcomprises, forming a dry aerosol comprising. the product of mixing dryair with the vapors from the addition of tin tetrachloride to drymethanol and flowing the dry aerosol through the tube while the latteris maintained under dry conditions and is at a temperature of from about500 to 700 C.

References Cited in the file of this patent UNITED STATES PATENTS2,429,420 McMaSter Oct. 21, 1947 2,570,245 .Tunge Oct. 9, 1951 2,612,615Fehr et al Sept. 30, 1952

10. AN IONIZATION DEVICE COMPRISING A RIGID TUBULAR ENVELOPE HAVING ANIMPERVIOUS INNER SURFACE COATED WITH A CLEAR, TRANSPARENT,MOISTURE-FREE, HALOGEN-RESISTANT, ELECTRICALLY CONDUCTIVE FILM OF AFINELY CRYSTALLINE METAL OXIDE, SAID FILM BEING A CATHODE AND OF INIFORMTHICKNESS AS SHOWN BY ITS SINGLE REFLECTANCE INTERFERENCE COLOR IN WHITELIGHT, AN ANODE OF SMALL DIAMETER AND AN IONIZABLE, HALOGEN-CONTAININGGAS FILLING IN SAID ENVELOPE.